What Happens During Meiosis That May Lead to a Monosomy or Trisomy?

Figure 2: Examples of polytene chromosomes

Pairing of homologous chromatids results in hundreds to thousands of individual chromatid copies aligned tightly in parallel to produce giant, "polytene" chromosomes.

© 2007 Nature Publishing Grouping Novikov, D. et al. High-force per unit area handling of polytene chromosomes improves structural resolution. Nature Methods 4, 483 (2007). All rights reserved.

Although he did not know it, Walther Flemming really observed spermatozoa undergoing meiosis in 1882, simply he mistook this procedure for mitosis. Withal, Flemming did notice that, unlike during regular cell partitioning, chromosomes occurred in pairs during spermatozoan development. This observation, followed in 1902 past Sutton's meticulous measurement of chromosomes in grasshopper sperm prison cell development, provided definitive clues that cell division in gametes was non just regular mitosis. Sutton demonstrated that the number of chromosomes was reduced in spermatozoan prison cell division, a procedure referred to as reductive division. Every bit a result of this process, each gamete that Sutton observed had half the genetic information of the original cell. A few years subsequently, researchers J. B. Farmer and J. E. S. Moore reported that this procedure—otherwise known as meiosis—is the fundamental ways by which animals and plants produce gametes (Farmer & Moore, 1905).

The greatest touch of Sutton'due south work has far more to practise with providing testify for Mendel's principle of independent assortment than annihilation else. Specifically, Sutton saw that the position of each chromosome at the midline during metaphase was random, and that there was never a consequent maternal or paternal side of the cell segmentation. Therefore, each chromosome was independent of the other. Thus, when the parent cell separated into gametes, the set of chromosomes in each girl cell could incorporate a mixture of the parental traits, but non necessarily the aforementioned mixture every bit in other daughter cells.

To illustrate this concept, consider the multifariousness derived from only three hypothetical chromosome pairs, every bit shown in the following example (Hirsch, 1963). Each pair consists of two homologues: one maternal and one paternal. Here, uppercase letters correspond the maternal chromosome, and lowercase messages represent the paternal chromosome:

• Pair 1: A and a
• Pair 2: B and b
• Pair 3: C and c

When these chromosome pairs are reshuffled through independent array, they tin can produce eight possible combinations in the resulting gametes:

• A B C
• A B c
• A b c
• A b C
• a B C
• a B c
• a b C
• a b c

A mathematical calculation based on the number of chromosomes in an organism will too provide the number of possible combinations of chromosomes for each gamete. In item, Sutton pointed out that the independence of each chromosome during meiosis means that there are iiⁿ possible combinations of chromosomes in gametes, with "n" being the number of chromosomes per gamete. Thus, in the previous example of 3 chromosome pairs, the calculation is 2ⁱⁱⁱ, which equals viii. Furthermore, when you consider all the possible pairings of male and female gametes, the variation in zygotes is (ii^{due north})², which results in some fairly large numbers.

Only what about chromosome reassortment in humans? Humans have 23 pairs of chromosomes. That means that i person could produce 2²³ different gametes. In improver, when you calculate the possible combinations that emerge from the pairing of an egg and a sperm, the effect is (2²³)^two possible combinations. However, some of these combinations produce the aforementioned genotype (for example, several gametes can produce a heterozygous individual). Every bit a result, the chances that two siblings will accept the same combination of chromosomes (assuming no recombination) is nearly (three/8)²³, or i in vi.27 billion. Of course, there are more than 23 segregating units (Hirsch, 2004).

While calculations of the random assortment of chromosomes and the mixture of different gametes are impressive, random assortment is not the merely source of variation that comes from meiosis. In fact, these calculations are ideal numbers based on chromosomes that actually stay intact throughout the meiotic process. In reality, crossing-over between chromatids during prophase I of meiosis mixes up pieces of chromosomes between homologue pairs, a phenomenon called recombination. Because recombination occurs every time gametes are formed, nosotros can expect that it will always add to the possible genotypes predicted from the 2^north calculation. In improver, the variety of gametes becomes even more than unpredictable and circuitous when we consider the contribution of gene linkage. Some genes will always cosegregate into gametes if they are tightly linked, and they will therefore prove a very low recombination rate. While linkage is a forcefulness that tends to reduce contained assortment of certain traits, recombination increases this assortment. In fact, recombination leads to an overall increment in the number of units that assort independently, and this increases variation.

While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction, genes get mixed up. Sexual reproduction actually expands the diverseness created by meiosis, because it combines the different varieties of parental genotypes. Thus, because of independent array, recombination, and sexual reproduction, in that location are trillions of possible genotypes in the human species.

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Source: http://www.nature.com/scitable/topicpage/mitosis-meiosis-and-inheritance-476